The main objectives of water treatment in order to make it drinkable in accordance with the standards in force are as follows:
eliminate suspended solids, PA0 eliminate organic materials, PA0 eliminate unwanted ions, PA0 sterilize.
Conventional treatment for achieving these objectives use a series of physicochemical steps including coagulation, flocculation, settlement, filtration, and usually oxidation.
The role of the filtration step, to which the invention relates particularly, is to disinfect treated water by retaining micro-organisms (viruses, bacteria and protozoa) contained in the water, and particularly pathogenic micro-organisms.
This membrane filtration step is advantageously carried out by means of organic membranes with variable size pore diameters depending on the size of the particles to be retained, and possibly with different configurations (hollow fibers, spiral modules, etc.).
Ultrafiltration and microfiltration with organic membranes are thus considered to be excellent methods of treating water and making it drinkable.
One of the main problems that arises with installation using membrane filtration is due to leaks that may occur in the membranes, significantly reducing their efficiency.
In practice, there are several potential sources of leaks in this type of installation using membranes, including particularly membrane imperfections, mechanical joints, joints and glue spots and membrane breakages. The problem of membrane breakages is more severe with membranes composed of hollow fibers that are relatively brittle.
Therefore in order to overcome this problem, particularly within the context of making water drinkable, it is essential to have processes capable of guaranteeing the integrity of membrane systems, and verifying that they do not leak. This type of process is intended to quickly locate leaks so that the defective elements responsible for the local leak can be repaired or replaced. It is essential that this type of process can be applied in situ, in other words directly on the filtration installation without needing to remove the filtration membranes.
The state-of-the-art includes several processes for achieving this objective.
Some processes simply consist of counting particles in the filtered liquid (permeate) in order to determine if the filtration operation is done correctly by the tested installation. In practice, if the number of particles found in the permeate is too high, it may be concluded that there is a leak in the installation. Although processes of this type are efficient, they have several disadvantages. Firstly, relatively sophisticated and expensive equipment necessary for particle counts has to be used. Secondly and especially, they have the disadvantage that they cannot be used on water with a low initial content of particles to be filtered.
Japanese patent application JP-A-H7024273 proposes to use a gas containing particles with a constant size at a constant concentration, to filter the gas in question through the membranes to be treated, and to detect particles on the permeate side. This technique has the disadvantage that a special fluid needs to be used, namely a gas containing particles with a constant composition, which increases the complexity and cost of the integrity test.
Japanese patent application JP-A-H7060073 purposes a technique consisting of installing a microfilter at the outlet of the main filtration installation, and from time to time measuring the pressure in this microfilter. Any pressure increase at the microfilter suggests that there must be a leak. The main disadvantage of this technique is that it requires the use of an additional filtration device that is relatively difficult to use and significantly increases the total cost of the installation.
Another method consists of using a hydrophore to detect noise resulting from the breakage of hollow fibres. However, this type of test can only detect leaks on membranes made with hollow fibers, in which air is used for backwashing.
Another suggestion in the state-of-the-art, and particularly in American patent application U.S. Pat. No. 5,353,630, suggests evaluating the integrity of filtration membranes using the bubble point principle. This measurement consists of wetting the membranes to be tested and submitting it to a gradually increasing air pressure until the air flushes the liquid through the leak orifices in the said membrane. By using test pressures between about 0.5 bars and 1 bar, it is thus possible to detect the presence of orifices with a size of the order of 1 micron corresponding to imperfections in the filter layer, leaking seals, broken hollow fibers, etc. The size of this type of leak orifice is considerably larger than the cutoff limits of tested membranes which are the order of 0.1 .mu.m for microfiltration membranes, 0.001 .mu.m for ultrafiltration membranes and even smaller for inverse osmosis.
The Young and Laplace equation can be used to estimate the sizes of these orifices allowing air to pass and thus determine whether or not there are any leaks in the membrane. According to this equation: EQU d=4.gamma. Kt cos.theta./.DELTA.P
where d is the orifice diameter, .gamma. is the surface tension at the air-liquid interface, Kt is a correction factor taking account of the tortuosity of the pores and which is typically equal to 0.2 to 0.3 for membranes made by phase inversion, .DELTA.P is the bubble point, and .gamma. is the surface tension at the air-liquid interface. Note that when an air bubble penetrates into an orifice, the diameter of this bubble reaches the diameter of the orifice and therefore .theta.=0 and cos.theta.=1.
U.S. Pat. No. 5,353,630 consists of applying air pressure to the upstream compartment delimited by the membrane and measuring the air flow representing the air flow passing through the membrane.
This technique has the disadvantage that the upstream compartment has to be pressurized, which leads to the need to equip the installation with means of supplying pressurized air. However means of pressurizing air are only present on some types of filtration installations, and particularly those that use backwashing of membranes by air.